Method of forming a partially perforated foam

ABSTRACT

A method for accelerating the release of a flammable blowing agent from an expanded foam is disclosed. According to the method, an expanded foam structure is perforated to form a first series of channels extending partially through its thickness from a first surface and a second series of channels extending partially through its thickness from a second surface opposite the first surface. The channels provide paths enabling the blowing agent to escape more rapidly from the interior of the expanded foam structure.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is a division of application Ser. No.09/238,989, filed Jan. 28, 1999, now U.S. Pat. No. 6,207,254.

FIELD OF THE INVENTION

The present invention relates to expandable foams, and more particularlyto extruded expandable foams, such as those used for packaging. Stillmore particularly, the present invention is directed to methods forproducing expanded foam structures so as to accelerate the release ofblowing agents therefrom.

BACKGROUND OF THE INVENTION

Expandable foam products, which find use as packaging, cushioning,insulating and structural materials, typically consist of a phase ofopen or closed pores or cells dispersed throughout a polymer matrix. Awide array of processes have been devised for developing the cell phasein these products, including adding a gaseous “blowing agent” to thepolymer during processing, producing a gaseous blowing agent by chemicalreaction within the polymer during processing, and forming the productfrom polymer granules to obtain a cellular structure. In oneparticularly popular process, a gaseous blowing agent is incorporatedinto a molten thermoplastic material to form a mixture which may then bemolded to a desired shape, such as by extrusion. After molding, appliedheat or reduced pressure causes the blowing agent to expand, forming acellular structure within the thermoplastic matrix. The effectiveness ofa particular blowing agent will depend largely upon the polymercomposition in which it is incorporated, the method of incorporation,the process conditions, the additives used, and the products sought.

Blowing agents work by expanding the polymer to produce a cellularstructure having far less density than the polymer itself. In processesin which a blowing agent is incorporated into a molten thermoplasticpolymer, bubbles of gas form around “nucleation sites” and are expandedby heat or reduced pressure. A nucleation site is a small particle or aconglomerate of small particles that promotes the formation of a gasbubble in the polymer. Additives may be incorporated into the polymer topromote nucleation for a particular blowing agent and, consequently, amore uniform pore distribution.

Once bubbles of the blowing agent have expanded to form the cellularstructure, the structure is maintained by replacing the blowing agent inthe cells with air. Diffusivity of the blowing agent out of the cellsrelative to air coming into the cells impacts the stability of the foamover time and whether the cells of the foam may collapse. Additives maybe incorporated into the polymer and process conditions may be adjustedto assist in controlling the diffusivity of the blowing agent, topromote foam stability, and to limit collapse of the foam to acceptablelimits.

Many methods are available for adding a blowing agent to a polymerduring processing to produce a foam. In one method pertinent to thepresent invention, the blowing agent is mixed with a moltenthermoplastic polymer under pressure, and the mixture is then extrudedthrough a forming die into a zone of reduced pressure. Shaped extrudedfoams may be produced by this method using a forming die of a desiredconfiguration. Plank, which can be cut to a desired shape, and thin foamsheets may also be produced in this manner.

Prior art processes for forming expanded foam products fromthermoplastic polymers typically used halogenated hydrocarbons asblowing agents. The halogenated hydrocarbons include thechlorofluorocarbons (“CFCs”) and hydrochlorofluorocarbons (“HCFCs”).CFCs and HCFCs are readily impregnable in thermoplastic polymers and arereadily expandable under relatively mild conditions. CFCs and HCFCsgenerally produce foams of high quality with a minimum of processingdifficulty. The pore size is controllable, the foam has good stabilitywith minimum tendency to collapse after a period of time, and thesurface characteristics of the foam are smooth and desirable. Also,CFCs, HCFCs and other halogenated hydrocarbons typically are either notflammable or are of low flammability, which greatly reduces the carewith which they may be used. These compounds have the further advantageof low toxicity. However, CFCs, HCFCs and other halogenated hydrocarbonshave been linked to ozone depletion in the atmosphere. As a result ofconcern over the ozone layer, the use of these materials is being phasedout in favor of materials which are more friendly to the ozone layer,such as hydrocarbons.

Although hydrocarbons are readily available, inexpensive and verycompatible with polyethylene and other polymer matrix materials, therebypermitting wide processing variability, they present their own uniqueproblems. Foremost among these problems is the greater flammability ofthese materials. Other problems with hydrocarbon blowing agents mayinclude toxicity or environmental incompatibility. Moreover, thehydrocarbon blowing agents are slow to permeate through the expandedfoam structure, such that the flammability and other problems associatedwith these materials persist in the foam structures for longer periodsof time. Safety concerns have therefore mandated that manufacturers ofthese products store them for excessively long periods of time to enablethe blowing agents therein to dissipate to levels below their lowestexplosive limit so that the products are safe enough to be shipped toand used by customers.

The problems associated with the use of hydrocarbon blowing agents wouldbe minimized if a majority of the blowing agent could be removed fromthe expanded foam structure as quickly as possible. Although attemptshave been made in the past to do just that, these attempts have provento be unsatisfactory. Thus, in U.S. Pat. Nos. 5,424,016 and 5,585,058 toKolosowski, an expanded foam structure is perforated with a multiplicityof channels extending from one surface of the structure to the oppositesurface. These channels shorten the path through which the blowing agentmust travel to be diffused from the interior of the structure to theatmosphere. However, these channels also decrease the mechanicalproperties of the foam, including its compression strength, resistanceto creep, cushioning ability and the like. U.S. Pat. No. 5,776,390 toFiddelaers et al. also teaches perforating an extruded foam in order tofacilitate the dissipation of the blowing agent. In this method,however, the perforations are made from one side of the foam and extendthrough only about 60-97 percent of the foam thickness so as to avoidremoval of the surface skin from the foam. The problem with thisapproach, however, is that perforating from a single side while the foamis still hot causes residual stresses to develop in the foam, thusresulting in foam warpage.

Despite the efforts that have been made in the past, there remains aneed for production methods which will accelerate the removal of amajority of the blowing agent from the expanded foam structure withoutdetrimentally affecting its resulting strength, cushioning properties oroverall foam quality.

SUMMARY OF THE INVENTION

The present invention addresses these needs.

One aspect of the present invention provides methods for acceleratingthe release of blowing agents from expanded foam structures. Inaccordance with these methods, an expanded foam structure is providedwith the blowing agent therein, the foam structure having first andsecond surfaces separated by a predetermined dimension. The foamstructure is perforated to form a first series of channels extendingfrom the first surface toward the second surface, and to form a secondseries of channels extending from the second surface toward the firstsurface. The first and second series of channels each may have a lengthof up to about 50% of the predetermined dimension. Preferably, the firstseries of channels extends substantially perpendicular to the firstsurface and the second series of channels extends substantiallyperpendicular to the second surface. The methods of the invention may beused to perforate foams having a thickness of about 12 mm or greater.

In preferred embodiments, the length of the first series of channels isbetween about 30% and about 50% of the predetermined dimension. In morepreferred embodiments, the length of the second series of channels isalso between about 30% and about 50% of the predetermined dimension.

Desirably, each of the channels in the first series of channels isspaced from an adjacent channel in the first series of channels bybetween about 1/12 and 1/2 of the predetermined dimension. Each of thechannels in the second series of channels preferably is spaced from anadjacent channel in the second series of channels by an amount withinthe same range. In highly preferred embodiments, each of the channels inthe second series of channels has a position offset in the length andwidth directions of the foam from the channels in the first series ofchannels.

In preferred methods, the expanded foam structure may be formed byextruding through a die a mixture including a polymer and a blowingagent, and the perforation procedure may be performed between about 40minutes and about 90 minutes after extrusion. Perforating at about 50minutes after extrusion is most preferred.

Another aspect of the present invention provides an expanded foamstructure comprising a body including a polymer matrix and a blowingagent dispersed in a multiplicity of cells throughout the matrix, thebody having first and second surfaces separated by a predetermineddimension; a first series of channels extending from the first surfacetoward the second surface, and a second series of channels extendingfrom the second surface toward the first surface, the first and secondseries of channels having a length of up to about 50 percent of thepredetermined dimension. Preferably, the first series of channelsextends substantially perpendicular to the first surface and the secondseries of channels extends substantially perpendicular to the secondsurface.

In preferred embodiments hereof, the length of the first series ofchannels is between about 30% and about 50% of the predetermineddimension, and most preferably, the length of the second series ofchannels is also between about 30% and about 50% of the predetermineddimension.

In further preferred embodiments hereof, each of the channels in thefirst series of channels is spaced from an adjacent channel in the firstseries of channels by between about 1/12 and about 1/2 of thepredetermined dimension. Desirably, each of the channels in the secondseries of channels is spaced from an adjacent channel in the secondseries of channels by an amount within the same range. In highlypreferred embodiments of the foam structures, each of the channels inthe second series of channels has a position offset in the length andwidth directions of the foam from the channels in the first series ofchannels.

BRIEF DESCRIPTION OF THE DRAWINGS

A more complete appreciation of the subject matter of the presentinvention and the various advantages thereof can be realized byreference to the following detailed description, in which reference ismade to the accompanying drawings, in which:

FIG. 1 is a highly schematic cross-sectional view of a foam structureprior to perforating;

FIG. 1A is an enlarged detailed view showing the gas cells dispersedthroughout the polymer matrix;

FIG. 2 is a highly schematic top plan view of a foam structureperforated in accordance with the present invention; and

FIG. 3 is a highly schematic cross-sectional view taken along line 3—3of FIG. 2.

BRIEF DESCRIPTION OF THE PREFERRED EMBODIMENTS

Preferred processes for forming expanded foams according to the presentinvention generally include the steps of (1) providing a mass of apolymer in a flowable state; (2) incorporating a blowing agent in thepolymer mass; (3) forming the polymer/blowing agent mixture to a desiredshape; (4) expanding the blowing agent to form a foam structureconsisting of a phase of pores or cells dispersed throughout a polymermatrix; (5) solidifying the foam structure; and (6) perforating the foamstructure to facilitate the release of the blowing agent therefrom.Polymers, additives and blowing agents useful in the present invention,as well as methods for combining these materials and forming same intoexpanded foam structures, are well known in the art and include thosedisclosed in commonly assigned U.S. Pat. No. 5,667,728 and pending U.S.patent application Ser. No. 08/940,366, filed Sep. 30, 1997, thedisclosures of which are hereby incorporated by reference herein.

The polymers which may be used in the present invention include anyfoamable thermoplastic or thermosetting materials, including blends oftwo or more thermoplastic materials, blends of two or more thermosettingmaterials, or blends of thermoplastic materials with thermosettingmaterials. Suitable polymers include polystyrene, polyolefins,polyurethanes, polyesters including polyethylene terephthalate, andpolyisocyanurates, with polyolefins being particularly preferred.Polyolefins are thermoplastic polymers derived from unsaturatedhydrocarbons containing the ethylene or diene functional groups.Although the polyolefins may include virtually all of the additionpolymers, the term polyolefin ordinarily refers to polymers of ethylene,the alkyl derivatives of ethylene (the alephaolefins), and the dienes.Among the more commercially important polyolefins are polyethylene,polypropylene, polybutene, and their copolymers, includingethylene/alpha-olefin copolymers such as linear low densitypolyethylene, and blends of the foregoing materials. Polyethylene isparticularly useful in the practice of the present invention.

Polyethylene is a whitish, translucent polymer of moderate strength andhigh toughness which is available in varieties ranging in crystallinityfrom 20 to 95 percent, and in ultra low, low, medium and high densitypolymer forms. The low density material has a softening temperaturebetween about 95° C. and about 115° C., while the high density materialhas a softening temperature between about 130° C. and about 140° C. Low,medium and high density polyethylenes and mixtures thereof are suitablefor extrusion forming.

The present invention may utilize any of the known blowing agents,including fluorocarbons; hydrofluorocarbons; chlorofluorocarbons;hydrochlorofluorocarbons; alkylhalides, such as methyl chloride andethyl chloride; and hydrocarbons. Other suitable blowing agents mayinclude pristine blowing agents such as air, carbon dioxide, nitrogen,argon, water and the like. The blowing agent may consist of a mixture oftwo or more of any of the blowing agents set forth above. Other suitableblowing agents may also include chemical blowing agents such as ammoniumand azotype compounds, including ammonium carbonate, ammoniumbiocarbonate, potassium biocarbonate, diazoaminobenzene,diazoaminotolulene, azodicarbonamide, diazoisobutyronitrile, and thelike.

Preferred blowing agents in accordance with the present invention arehydrocarbons, including butane, isobutane, pentane, isopentane, hexane,isohexane, heptane, propane and the like, including combinations of twoor more of these materials. A particularly preferred blowing agent foruse with polyethylene is isobutane.

Although the blowing agent may be flammable or nonflammable, the methodsof the present invention are particularly useful with flammable blowingagents because of the accelerated release of the blowing agent that theyprovide. As used herein, a flame blowing agent is one that has a lowestexplosive limit as determined by the ASTM 681-85 test. Flammable blowingagents include the alkylhalides, alkanes and alkenes described above.

In addition to the polymer and the blowing agent, the mixtures forforming the foam structures of the present invention may include one ormore additives for enhancing the properties of the foam and the formingprocess. For example, elastomeric components such as polyisobutylene,polybutadene, ethylene/propylene copolymers, and ethylene/propylenediene interpolymers may be incorporated in the mixture. Other potentialadditives include crosslinking agents, extrusion aids, antioxidants,colorants, pigments, etc. as desired, all of which are conventional inthe art.

The mixture may also include one or more permeability modifiers forcontrolling the replacement of the blowing agent in the cells of thefoam with air while preventing substantial shrinkage of the foamstructure from premature excessive loss of the blowing agent. Suitablepermeability modifiers include fatty acid esters and amides such asglycerol monostearate and stearyl stearamide. The permeability modifieris used in an amount sufficient to produce a desirable rate of exchangeof air with blowing agent in the cells of the foam. This amount isgenerally dictated by the polymer matrix material, the blowing agentcomposition and quantity, processing conditions, etc. For mixtures inwhich the polymer is polyethylene and the blowing agent is isobutane, aglycerol monostearate permeability modifier may be mixed with thepolyethylene, preferably prior to melting, in an amount from about 0.3to about 5 percent by weight of the polyethylene. Glycerol monostearateadditions of about 0.3 to about 1.5 percent by weight of thepolyethylene are preferred. On the other hand, where the polymer ispolypropylene, permeability modifiers are not typically needed, but maybe used in appropriate quantities to reduce friction induced static inthe polymer and foams.

A nucleation agent may also be added to the mixture to promotenucleation and to control cell development and size. Preferrednucleating agents include low activity metal oxides, such as zinc oxide,zirconium oxide and talc; sodium bicarbonate/citric acid blends, such asthose available under the trademark Hydrocerol from Boehringer Ingelheimof Winchester, Va.; and other materials known in the art. The amount ofnucleating agent added to the mixture will depend upon the compositionand activity of the nucleating agent, the composition of the polymermatrix material, the blowing agent composition and quantity, andprocessing conditions, as well as upon the pore size and pore densitydesired in the expanded foam. Sodium bicarbonate/citric acid nucleatingagents preferably are added in an amount of from about 0 to 0.8 percentby weight of the polymer. For polyethylene/isobutane mixtures, additionsof such nucleating agents in amounts between about 0.05 and 0.50 percentby weight of polyethylene are preferred. Talc, which is less active, ispreferably added as a nucleating agent in an amount of from about 0 to 2percent by weight of the polymer, with additions of between about 0.2and 1.0 percent by weight of polyethylene being preferred inpolyethylene/isobutane mixtures.

Once the polymer and additives have been selected, these materials aremixed together to form a mixture. This may be accomplished in aconventional batch mixing step. Alternatively, where the foam structuresare to be formed by extrusion, pellets of a thermoplastic polymer may beplaced in the hopper of an extruder. Any nucleating agents, permeabilitymodifiers and/or other additives may be added to the hopper and combinedin a solid state with the polymer pellets to form a homogenous mixture.Intimate mixing of these components is important to assure uniform poredistribution throughout the extruded foam as well as uniform blowingagent dissipation from the expanded foam. The solid mixture may then beconveyed to the melt zone of the extruder in which the mixture isthoroughly melted. The mixture should be brought to a high enoughtemperature above its melting point to have sufficient fluidity formixing with the blowing agent. Temperatures which are between about 20°C. and about 100° C. above the melting point of the polymer arepreferred. The melt zone may be maintained at a somewhat lowertemperature due to the heat that is generated by friction as the meltedmixture flows through the extruder.

The melted mixture may then be metered to a mixing zone where it ismixed with the blowing agent under pressure. The blowing agent typicallyis injected between the metering and mixing zones, and through either asingle port or multiple ports, using high pressure pumps. Where theblowing agent includes more than one component, the components may beinjected separately through multiple ports or in combination through asingle port. When injected, the blowing agent initially forms adispersion of insoluble bubbles within the melted thermoplastic mass.These bubbles eventually dissolve in the thermoplastic mass as themixing continues and the pressure increases down the length of theextruder. Desirably, the extruder has a length to diameter ratio of atleast 30:1 and a mixing zone with a sufficient length to ensure that ahomogenous mixture is formed. In this regard, single screw extruders maybe used in processes according to the present invention, although doublescrew extruders may be used for greater mixing. Double screw extrudersmay be either twin screw, in which the mixture passes through two screwsarranged parallel to one another, or tandem screw, in which the mixturepasses through two screws arranged in series.

The blowing agent generally is added to the molten polymer in amounts ofbetween about 5 and about 15 percent by weight of the polymer. Forpolyethylene/isobutane mixtures, the isobutane preferably is added inamounts of between about 6 and about 10 percent by weight ofpolyethylene for thicker grades of foam. Where lower density foams aredesired, greater amounts of blowing agent are typically added. Thus, themaximum useful proportion blowing agent in the molten mass is densityrelated. The quantity of blowing agent also is related to the pressurethat is maintained on the molten polymer/blowing agent mixture in theextrusion die passage.

After mixing, the temperature of the polymer/blowing agent mixtureshould be lowered to a temperature which is closer to its melting pointso that the blowing agent does not readily escape from the polymer uponexpansion, thereby enabling the polymer to maintain its structure uponfoaming. However, overcooling of the mixture may hinder completeexpansion of the foam, and therefore should be avoided. The blowingagent has a plasticizing effect on the polymer mixture, reducing itsviscosity or resistance to flow, and so the melting point of thepolymer/blowing agent mixture ordinarily is below that of the polymeralone. The expansion temperature, which is above the melting point ofthe polymer/blowing agent mixture, may be empirically determined anddepends upon the composition of the polymer, the length of the extruderscrew, whether single or double screws are used, and on the compositionand amount of the blowing agent. For a low density polyethylene, theexpansion temperature generally will be in the range of between about85° C. and about 120° C.

When cooled to the appropriate temperature, the polymer/blowing agentmixture may be extruded through a shaped die having dimensions selectedto produce an expanded foam sheet or plank having desired dimensions.Within the die, the mixture is under high pressure which prevents thefoam from expanding. As it exits the die, however, the mixture isexposed to a low pressure environment, such as atmospheric pressure.This sudden drop in pressure causes bubble expansion or foaming of thestructure. An expanded foam structure 10 existing at this point in theprocess is shown in FIG. 1. Foam structure 10 has a top surface 20, abottom surface 30 generally parallel to the top surface, and a pluralityof pores or cells 40 dispersed throughout the polymer matrix.Immediately after cell formation, cells 40 are filled almost entirelywith the blowing agent, in this case isobutane.

In order to facilitate the diffusion of the blowing agent from cells 40,the present invention follows the foam expansion step with a coolingstep and then a perforation step. Cooling generally occurs under ambientconditions at least until the polymer in the foam has solidifiedsufficiently so that the polymer matrix is not overly distorted uponperforation. In the perforation step, the foam structure is punctured toform a multiplicity of channels 50 extending partially across thestructure from surface 20 toward surface 30, and partially across thestructure from surface 30 toward surface 20. A perforated foam structurein accordance with the present invention is shown in FIGS. 2 and 3.Perforation may be accomplished by puncturing the foam with amultiplicity of pointed, sharp objects in the nature of needles, pins,spikes, nails or the like. This may be accomplished in a continuousprocess in which the needles or other sharp objects are arranged in apredetermined pattern on the surfaces of a pair of opposed rollers. Theexpanded foam may be guided between the rollers, whereupon the rotationof the rollers causes the needles to penetrate surfaces 20 and 30 of thefoam to form channels 50 therein. This process may result in enlargingof channels 50 as a result of the angular movement of the needlesrelative to the foam. In an alternate process, a pair of plates, onearranged on each side of the foam, may include one or more rows ofneedles. The plates may be moved toward surfaces 20 and 30 of the foameither simultaneously or alternately to form channels 50 within thefoam. To minimize distortion of the channels as the foam is beingperforated, the plates of needles may be driven in the movementdirection of the foam at the same speed as the foam is traveling. Uponretraction from the foam, the plates of needles may be moved in anopposite direction back to the starting position.

Channels 50 may have any cross-sectional shape, including circular,oval, square, rectangular or other polygonal configuration. Preferably,channels 50 have a diameter or corresponding cross-sectional size ofbetween about 0.01 and about 3.0 mm, and are uniformly dispersed oversurfaces 20 and 30, with the spacing between adjacent channels beingbased upon the foam thickness, t. Channel spacings of between about t/12and about t/2 on each side of the foam are preferred.

The depth of channels 50 is an important feature of the presentinvention. Thus, channels 50 preferably have a depth between about 25percent and about 50 percent of the thickness of foam 10. The depth ofchannels 50 may be controlled by controlling the depth to which theneedles penetrate the foam. This may be accomplished by controlling thelength of the puncturing needles. Penetration depth may also becontrolled, in the case of opposed rollers, by adjusting the distancebetween the rotation axes of the rollers, and in the case of opposedneedle plates, by controlling the amount of movement of the platestoward foam surfaces 20 and 30.

Although channels 50 are depicted in the figures as being orientedsubstantially perpendicular to the extrusion or elongation direction offoam 10, this need not be the case. Thus, channels 50 may be formed atan oblique angle relative to the extrusion direction. Angles of betweenabout 30° and about 120° relative to the extrusion direction arepreferred, with angles of about 90° being most preferred.

As noted above, the perforation step is conducted after a cooling stepwhich is sufficient in length to enable the foam structure to achieveadequate internal stability. If the foam is perforated too soon, theelastic nature of the polymer may cause the cells opened by theperforation procedure to close. On the other hand, if too long a periodof time elapses before perforation, the perforation procedure may have amore damaging affect on the foam properties adjacent the puncture sites.Preferably, the perforation procedure is performed between about 40minutes and about 90 minutes after initiation of free foam expansion.Performing the perforation step at about 50 minutes after the initiationof free foam expansion is most preferred.

Certain features of the present invention as described above areillustrated in the following examples.

EXAMPLE 1

A low density polyethylene resin having a melt index of 2 and a densityof 0.918 g/cm³(Novacor 219a available from Nova of Calgary, Alberta,Canada) was combined in the feed zone of the primary extrusion chamberin a tandem extruder with 0.2 wt % of a sodium bicarbonate/citric acidnucleating agent available from Boehringer Ingelheim under the nameHydrocerol CF20. The components were intimately mixed to form ahomogenous polymer mixture. The mixture was then conveyed to the meltzone of the extruder and heated to a temperature of about 190° C. toform a molten mass. About 6-7 wt % (based on the weight of the resin) ofan isobutane blowing agent was injected into the molten mass using ametering and pumping unit, and mixing continued to form a homogenousblowing agent/polymer mixture. This mixture was then conveyed to thesecondary extrusion chamber in which it was cooled to a temperature ofabout 115° C. Once stabilized, the mixture was extruded through a dieand expanded to form a foam plank having a thickness of about 5.4 cm anda width of about 134 cm. The foam plank was then conveyed under ambientconditions to a cutting station and cut into lengths of about 2.8meters. After predetermined cooling periods, the foam planks wereperforated with needles to form channels generally circular incross-section with a diameter of about 1 mm. Perforations were madethrough about 50 percent of the foam thickness from one side, throughabout 50 percent of the foam thickness from two sides, through about 75percent of the foam thickness from one side, and entirely through thethickness of the foam. The channels were formed in a square patternevery 0.8 cm, with the channels on one side of the foam plank offset inthe length and width directions from the channels on the other side ofthe foam plank by one-half of the distance between the channels so thateach channel on one side of the foam plank was positioned inapproximately the center of a square formed by four channels on theother side of the foam plank.

After a period of 14 days following foam production, each foam samplewas tested to determine its density, compressive strength and residualgas content. Compressive strength measurements were made to determinethe force required to compress the foam by 25 percent and 50 percent ofits initial thickness both before and after die cutting (ADC). In theADC procedure, the foam plank was first compressed to about 20 percentof its initial thickness to simulate a die cutting procedure and thenreleased. Measurements were then made to determine compressive strengthat 25 percent compression and 50 percent compression, whichever had beenperformed initially. Retention is determined by dividing the compressivestrength after die cutting by the compressive strength before diecutting, with higher retention values being preferred. Residual gasmeasurements were made at approximately the center of the foam planksusing a hand held hydrocarbon sniffer with a stick nozzle inserted tothe point of measurement. Tables 1-3 below show the test results forsamples perforated at 30 minutes, 50 minutes and 120 minutes,respectively, after free foam expansion.

TABLE 1 50% 50% 75% Depth of perforation 0 (1 side) (2 sides) (1 side)100% Time to perforation (min) N/A 30 30 30 30 Density (lb/ft³) 2.322.29 2.29 2.27 2.30 Compressive strength (psi) 25% 12.1 13.2 12.8 12.710.2 ADC 9.3 9.5 8.9 9.5 6.5 Retention 77% 72% 70% 75% 64% 50% 20.7 22.121.1 21.4 18.4 ADC 18.5 18.6 17.2 18.6 14.8 Retention 89% 84% 82% 87%80% Residual gas (vol %) 10 10 5 5 1.2

TABLE 2 50% 50% 75% Depth of perforation 0 (1 side) (2 sides) (1 side)100% Time to perforation (min) N/A 50 50 50 50 Density (lb/ft³) 2.242.20 2.20 2.20 2.26 Compressive strength (psi) 25% 11.9 12.9 12.5 12.310.0 ADC 9.2 9.8 9.3 9.5 7.3 Retention 77% 76% 75% 78% 73% 50% 20.4 21.520.7 20.9 18.1 ADC 18.4 19.0 18.0 18.6 15.1 Retention 90% 89% 87% 89%84% Residual gas (vol %) 8 11 5 5 1

TABLE 3 50% 50% 75% Depth of perforation 0 (1 side) (2 sides) (1 side)100% Time to perforation (min) N/A 120 120 120 120 Density (lb/ft³) 2.242.20 2.21 2.21 2.28 Compressive strength (psi) 25% 12.0 12.7 12.5 12.19.5 ADC 9.3 9.6 9.1 9.3 7.1 Retention 78% 76% 73% 77% 75% 50% 20.5 21.220.6 20.8 17.8 ADC 18.5 18.7 17.6 18.4 15.0 Retention 90% 88% 85% 88%84% Residual gas (vol %) 8 10 5 7 1

As shown in Tables 1-3, regardless of the time at which perforation wasperformed, the compressive strength of the foam planks perforatedhalfway through their thickness from both sides was only marginally lessthan the compressive strength of the foam planks perforated halfwaythrough their thickness from one side, yet yielded substantially lowerresidual gas volumes. Furthermore, perforating 50 percent through thethickness of the foam planks from both sides, while yielding somewhathigher residual gas volumes, also yielded significantly highercompressive strengths before and after a die cut procedure at both 25percent and 50 percent compression levels than were achieved whenperforations were made entirely through the thickness of the foam plank.

EXAMPLE 2

The same materials and substantially the same method as described inExample 1 were used to make a higher density foam plank. Thus, Novacor219a polyethylene resin was mixed in a tandem extruder with 0.3 wt % ofHydrocerol CF20 nucleating agent. After melting at a temperature ofabout 190° C., the polymer was combined with about 5 wt % isobutaneblowing agent to form a homogenous mixture. The mixture was cooled to atemperature of about 115° C. and extruded to form a foam plank having athickness of about 5.4 cm and a width of about 66 cm. After being cut tolengths of about 2.8 meters and cooled for predetermined lengths oftime, the planks were perforated in the manner described above.

At a period of 14 days following foam production, the density,compressive strength and residual gas content of the foam samples weredetermined. Tables 4-6 below show the test results for samplesperforated at 30 minutes, 50 minutes and 120 minutes, respectively,after free foam expansion.

TABLE 4 50% 50% 75% Depth of perforation 0 (1 side) (2 sides) (1 side)100% Time to perforation (min) N/A 30 30 30 30 Density (lb/ft³) 4.7 4.644.62 4.65 4.61 Compressive strength (psi) 25% 16.9 18.8 18.7 19.0 16.7ADC 12.3 12.9 12.6 13.4 12.1 Retention 73% 69% 68% 71% 72% 50% 28.3 30.330.0 31.7 29.1 ADC 24.6 24.5 23.8 25.4 23.4 Retention 87% 81% 79% 80%80% Residual gas (vol %) 15 11 5 6 5

TABLE 5 50% 50% 75% Depth of perforation 0 (1 side) (2 sides) (1 side)100% Time to perforation (min) N/A 50 50 50 50 Density (lb/ft³) — 4.74.68 4.68 4.64 Compressive strength (psi) 25% — 18.8 19.5 18.7 16.5 ADC— 12.7 13.4 13.1 11.5 Retention — 67% 69% 70% 70% 50% — 30.7 31.7 31.628.9 ADC — 24.4 25.2 24.9 22.2 Retention — 80% 80% 79% 77% Residual gas(vol %) — 11 8 5 5

TABLE 6 50% 50% 75% Depth of perforation 0 (1 side) (2 sides) (1 side)100% Time to perforation (min) — 120 120 120 120 Density (lb/ft³) — 4.664.64 4.66 4.69 Compressive strength (psi) 25% — 19.4 20.6 19.4 17.1 ADC— 12.8 12.8 12.8 12.2 Retention — 66% 62% 66% 71% 50% — 31.5 32.9 31.829.5 ADC — 24.2 23.8 24.2 23.8 Retention — 77% 72% 76% 81% Residual gas(vol %) 13 6 5 2

As shown in Tables 4-6, very little difference was found between thecompressive strength of the foam planks perforated halfway through theirthickness from both sides and the compressive strength of the foamplanks perforated halfway through their thickness from one side,regardless of the amount of cooling before perforation. Nonetheless,foam planks perforated from both sides yielded significantly lowerresidual gas volumes than planks perforated halfway through theirthickness from one side. Additionally, foam planks perforated halfwaythrough their thickness from both sides, although exhibiting higherresidual gas volumes, yielded consistently higher compressive strengths,both before and after die cutting, than foam planks perforated entirelythrough their thickness. It should also be noted that, for these higherdensity foams, planks perforated from one side, whether through 50% or75% of their thickness, showed a tendency to warp when fully cooled.

Although the invention herein has been described with reference toparticular embodiments, it is to be understood that these embodimentsare merely illustrative of the principles and applications of thepresent invention. It is therefore to be understood that numerousmodifications may be made to the illustrative embodiments and that otherarrangements may be devised without departing from the spirit and scopeof the present invention as defined by the appended claims.

We claim:
 1. A method for providing accelerated release of a blowingagent from an expanded foam structure, comprising providing an expandedfoam structure having first and second surfaces separated by apredetermined dimension, said foam structure having said blowing agenttherein; perforating said foam structure to form a first series ofchannels extending from said first surface toward said second surface,said first series of channels having a length of up to about 50 percentof said predetermined dimension; and perforating said foam structure toform a second series of channels extending from said second surfacetoward said first surface, said second series of channels having alength of up to about 50 percent of said predetermined dimension, saidsecond series of channels being formed so as to not connect with saidfirst series of channels.
 2. The method as claimed in claim 1, whereinsaid first series of channels extends substantially perpendicular tosaid first surface and said second series of channels extendssubstantially perpendicular to said second surface.
 3. The method asclaimed in claim 1, wherein said length of said first series of channelsis between about 30 percent and about 50 percent of said predetermineddimension.
 4. The method as claimed in claim 3, wherein said length ofsaid second series of channels is between about 30 percent and about 50percent of said predetermined dimension.
 5. The method as claimed inclaim 1, wherein each of said channels in said first series of channelsis spaced from an adjacent channel in said first series of channels bybetween about 1/12 and about 1/2 of said predetermined dimension.
 6. Themethod as claimed in claim 5, wherein each of said channels in saidsecond series of channels is spaced from an adjacent channel in saidsecond series of channels by between about 1/12 and about 1/2 of saidpredetermined dimension.
 7. The method as claimed in claim 1, whereineach of said channels in said second series of channels has a positionoffset in the length and width directions of said foam structure fromsaid channels in said first series of channels.
 8. The method as claimedin claim 1, wherein said step of providing said expanded foam structureincludes the step of extruding a mixture including a polymer and saidblowing agent through a die, and wherein said perforation step isperformed between about 40 minutes and about 90 minutes after saidextrusion step.
 9. The method as claimed in claim 8, wherein saidperforation step is performed about 50 minutes after said extrusionstep.